Since the invention of nasal Continuous Positive Airway Pressure (nasal CPAP) for treatment of Obstructive Sleep Apnea (OSA) and other forms of Sleep Disordered Breathing (SDB) by Sullivan, as taught in U.S. Pat. No. 4,944,310, much effort has been directed towards improving the comfort of the devices. One aspect of this is a more comfortable patient interface, such as provided by the MIRAGE® and ULTRA MIRAGE® masks manufactured by ResMed Limited. Another aspect of providing a more comfortable patient interface is the comfort of the waveform of air at positive pressure provided by the blower.
Some low cost CPAP blower devices, such as the S7™ device by ResMed Limited, provide a supply of air at a generally fixed positive pressure throughout the respiratory cycle of the patient, for example, 15 cmH2O. A blower comprising an electric motor and fan can be constructed to deliver air based on a rotational speed of the motor predetermined to deliver a particular pressure to a patient interface, such as a mask. When the patient breathes in with such a system, the pressure in the mask may reduce by a small amount. When the patient breathes out with such a system, the pressure in the mask may increase by a small amount. These fluctuations in mask pressure are referred to as “swing”. Other blowers use feedback in a pressure controller which counterbalances the effect of patient effort on the mask pressure to reduce the swing.
Another group of CPAP devices, such as the ResMed AUTOSET® SPIRIT™ device can monitor the patient and determine an appropriate CPAP setting to deliver to the patient, which pressure may vary through the night, for example, delivering 15 cmH2O during an initial portion of the patient's sleep, but increasing to 20 cmH2O later in the night. Changes in pressure are made in response to a determination of the occurrence and severity of aspects of breathing such as flow limitation and snoring.
A bi-level CPAP device, such as the ResMed VPAP® product, provides a higher pressure to the patient's mask during the inspiratory portion of the respiratory cycle, for example, 10-20 cmH2O, and a lower pressure during the expiratory portion of the patient's breathing cycle, for example, 4-10 cmH2O. When the device makes a transition from the higher pressure to the lower pressure the motor is braked. When the device makes the transition from the lower pressure to the higher pressure, the motor is accelerated. A mismatch between the device control cycle and the patient respiratory cycle can lead to patient discomfort.
U.S. Pat. No. 6,345,619 (Finn) describes a CPAP device that provides air at a pressure intermediate the IPAP (Inspiratory Positive Airway Pressure) and EPAP (Expiratory Positive Airway Pressure) pressures during the transition between the inspiratory and expiratory portions of the device control cycle. U.S. Pat. No. 6,484,719 (Berthon-Jones) and U.S. Pat. No. 6,532,957 (Berthon-Jones) describe devices which provide pressure support in accordance with a waveform template. U.S. Pat. No. 6,553,992 (Berthon-Jones et al.) describes a ventilator whose servo-controller adjusts the degree of support by adjusting the profile of the pressure waveform as well as the pressure modulation amplitude. As the servo-controller increases the degree of support by increasing the pressure modulation amplitude, it also generates a progressively more square, and therefore efficient, pressure waveform; when the servo-controller decreases the degree of support by decreasing the pressure modulation amplitude, it also generates a progressively more smooth and therefore comfortable pressure waveform. The contents of all of these patents are hereby incorporated by reference.
CPAP and VPAP devices are mechanical ventilators. Ventilators have been classified (Chatburn, Principles and Practice of Mechanical Ventilation, Edited by M J Tobin, McGraw Hill, 1994, Ch. 2) as being either pressure, volume or flow controllers. In each case, the ventilator controls the pressure of air versus time, volume of air versus time, or flow of air versus time that is delivered to the patient. Many such devices can be programmed to deliver a variety of waveforms, such as pulse (rectangular), exponential, ramp and sinusoidal. The shape of the waveform actually delivered to the patient may be affected by the compliance and resistance of the patient's respiratory system and his breathing effort, as well as mechanical constraints such as blower momentum and propagation delays.
Ventilators have been constructed to deliver an inspiratory waveform when one of pressure, volume, flow or time reaches a preset value. The variable of interest is considered an initiating or trigger variable. Time and pressure triggers are common. The Puritan Bennett 7200a ventilator is flow triggered. The Drager Babylog ventilator is volume triggered. The Infrasonics Star Sync module allows triggering of the Infant Star ventilator by chest wall movement. The ventilator's inspiration cycle ends because some variable has reached a preset value. The variable that is measured and used to terminate inspiration is called the cycle variable. Time and volume cycled ventilators are known.
Many ventilators provide a Positive End-Expiratory Pressure (PEEP). Some of these ventilators use a valve (the PEEP valve) which allows the PEEP to be varied. Some devices, such as that taught by Ernst et al. in U.S. Pat. No. 3,961,627, provide a combination of pressure and flow control within one respiration cycle. A control cycle is divided into four phases I, II, III and IV. The respiration cycle and the control cycle do not necessarily have to fall together in time; mostly, however, phases I and II of the control cycle correspond to inspiration, and phases III and IV of the control cycle correspond to expiration. Phases I, III and IV are pressure-regulated, and phase II is flow-regulated. The doctor can choose the pressure course with the three control elements for the expiratory pressure decrease, the inflexion, and the final expiratory pressure. In phase III, the pressure proceeds from the pressure measured at the end of phase II according to a fixed pressure decrease dP/dt. When the pressure measured in phase III reaches the inflexion, the pressure proceeds linearly to the fixed final expiratory pressure. The part of the expiration from the inflexion to the end of the respiration cycle represents phase IV. The linear course of the pressure in the expiration represents a preferred embodiment, but could be replaced by another course of the pressure curve, for example, an exponential.
A spontaneously breathing patient exerts at least some effort to breath, however inadequate. A lack of synchrony between the respiratory cycle of the patient and that of the ventilator can lead to patient discomfort.
In Proportional Assist Ventilation (PAV), as described by Magdy Younes, the ventilator generates pressure in proportion to patient effort; the more the patient pulls, the higher the pressure generated by the machine. The ventilator simply amplifies patient effort without imposing any ventilatory or pressure targets. It is understood that the objective of PAV is to allow the patient to comfortably attain whatever ventilation and breathing pattern his or her control system sees fit. The PAV system is further discussed in U.S. Pat. Nos. 5,044,362, 5,107,830, 5,540,222 and 5,884,662.
U.S. Pat. No. 5,535,738 (Estes et al.) describes a further PAV apparatus.
Another technique for improving patient comfort is disclosed in copending U.S. application Ser. No. 10/871,970 in the name of Farrugia et al. filed on Jun. 18, 2004. In this method, upon detection of the transition from inspiration to expiration, the blower motor is allowed to freewheel.
Despite the many approaches that have been taken to improve patient comfort, patients receiving CPAP therapy frequently complain of difficulty in exhaling, particularly at higher CPAP pressures. The work of breathing is increased by CPAP. The expiratory reserve volume during CPAP is higher than when CPAP is not present, which is unpleasant for many patients. Subjectively it is hard to breathe out, though easy to breathe in. Bilevel ventilation does not necessarily alleviate these problems. In bilevel ventilation, there are typically delays in detecting the onset of inspiration and then in delivering the desired increase in pressure to the inspiratory level. Suppose, for example, that a certain mask pressure is necessary to prevent upper airway obstruction at peak inspiratory flow, and the bilevel inspiratory positive airway pressure (IPAP) is set to equal this pressure. Then because of the above-mentioned delays, it may be the case that the pressure delivered is less than IPAP at the time of peak inspiratory flow, resulting in upper airway obstruction. More generally, the pressure required to prevent upper airway obstruction varies during the respiratory cycle, and the delay in delivering IPAP may result in a pressure below that required to prevent obstruction, more probably earlier in inspiration than at the time of peak inspiratory flow, when the difference between the actual pressure and that required to prevent obstruction is larger.
In order to prevent this, it may be necessary to set IPAP to a level somewhat above the pressure required to prevent obstruction at peak inspiratory flow (which is probably the optimum level), and it is not easy to determine how high this pressure should be. In particular, a reasonably optimal IPAP cannot be easily determined from the results of a previous CPAP titration. Of course, setting the bilevel expiratory positive airway pressure (EPAP) to the CPAP level determined from a previous titration will prevent obstruction, but will yield an IPAP significantly above that CPAP level. Under these circumstances, the bilevel ventilator will perform some of the work of breathing, but deliver pressures significantly above those actually required.
There is a need for an improved method that provides expiratory pressure relief, removes the possibility of runaway pressure falls during expiration, and ensures that the pressure during inspiration is sufficient to prevent airway collapse, with the parameters determining the pressure level (if not set automatically) being based on the results of a previous CPAP titration.